1. Field Of The Invention
The present invention relates to devices for determining the right time for delivering a stimulation pulse to a heart.
2. Description of the Prior Art
The ability to control heart rhythm with the aid of electrical stimulation is important to the well-being and survival of many people suffering from various heart defects. Battery-powered stimulation devices are available for different kinds of disorders, and they emit electrical impulses to trigger heart contractions, enabling the heart to perform its vital, blood-pumping function. The times at which stimulation pulses are delivered to a heart are determined in prior art automatic devices, in cases in which the heart retains at least partially normal electrical function, with the aid of some more or less easily measurable electrical unit, such as the chronological development of electrical voltage, in characteristic segments, measured between a stimulation electrode, or a separate sensor electrode, and the metal enclosure in which the stimulation device's battery and electrical control circuits are usually housed.
The electrical voltage developed by a heart has the following general pattern during a heart cycle. A low voltage pulse, the P wave, reflects electrical activity in the heart's atria when muscle cells in their walls contract. A more complex pulse segment is referred to as the QRS complex and comprises e.g. a large electrical pulse. This segment reflects electrical activity in the heart's ventricles when muscle cells contract in ventricular walls, thereby initiating and performing the heart's actual blood-pumping. Another low voltage pulse, indicating that cells in the ventricular walls are starting to recover from preceding contraction, is referred to as the T wave. These pulses/pulse segments normally follow each other in time, i.e. a P wave comes first in a heart cycle, followed by the QRS complex with the T wave last. However, if the QRS complex, for example, is absent or fails to appear at the right time, while a correct P wave is detected, the time at which a stimulation pulse is emitted for the purpose of triggering ventricular contraction and, accordingly, the heart's blood-pumping, can be determined from the time at which the P wave was detected.
An appropriately selected or advantageous time for emitting a stimulation pulse to trigger contraction of the heart's ventricles and ejection of blood for oxygenation and into the circulatory system can generally be regarded as the point at which sufficient blood has had time to flow into the ventricles so as to be pumped out. This can also be put another way, i.e. a stimulation pulse should be delivered as soon as the heart's ventricles contain sufficient blood. This makes the heart's blood-pumping effective and does not impose any needless mechanical strain on heart muscle, since this muscle only needs to remain in the stretched state for a brief period of time.
Determination of this appropriate time can be made from the electrical impedance measured between a stimulation electrode in a ventricle and some other electrode in the body or in the heart. This impedance is related, in a complex way, to the amount of blood in the ventricle, a circumstance utilized in U.S. Pat. No. 5,417,715, which is attached here as, and can be used for detecting the degree to which the ventricle of the heart is full. Thus, this patent depicts heart stimulation at a variable stimulation interval, the time for emitting a stimulation pulse being governed by the impedance, measured between an end electrode and a ring electrode, both of which installed in the same ventricle. The minimum of the curve of the impedance tracing is identified, and a stimulation pulse is emitted, after a pre-defined delay, when the curve subsequently increases to a given percentage of the aforementioned minimum. One disadvantage of this method is that correspondence between impedance and the degree of filling is not always unequivocal due to the fact that impedance measurements primarily reflect conditions close to the measurement site, e.g. around the electrode tip.